Re: [udf-dlm-develop] Some questions

[Joerg-Micha J. <jm...@Or...>]

Good New Year everybody. I am slowly catching up on e-mail, sorry about
the delay. 

To (attempt to) answer some of Harald's questions:


On Mon, 3 Jan 2000, Harald Frey wrote:
> 
> 1. Our WIC UDF contains the 256x256 image and several more single entries like 
> the start_time, high_voltage etc. The command udf_read returns all these single 
> numbers as 256-element arrays. Why? Is there a way around? I can certainly deal 
> with the fact that I only consider element[0] as valid, but I consider this  
> unnecessary additional effort.
> 

I believe you store your images as "matrix", which is not implemented yet.
If I understand this correctly, the code currently picks up only the first
column of your picture array (this is in the VIDF/PIDF; hence a 256
element vector), yet I fail to understand why only the first entry is valid. 

To fix this we need to add the "matrix". Now, the trick is that the number
of columns is stored in the header file, *not* the VIDF. This has to do
with the original definition of VIDF, I believe, and one possible way out
was to put it into the header. So in order to access the # of columns and
malloc things correctly, one first has to read_drec, recover the
appropriate number, and then read the data (info thanks to Carrie,
hopefully I didn't loose much in the translation). I realize this might
be easier said then done.


> 2. Is it possible to get the number of records in a UDF file? Ed's example reads 
> all records until the end of the file, but maybe there is a faster way?
> 

Dunno this one yet, sorry.

> 3. The udf_read procedure returns the complete structure of the record. Is it 
> possible to read only one particular variable, something like
> 
> d=udf_read(fh,variable='WIC_MCP_VOLTAGE_MONITOR')
> 


Well: "no" right now, "yes" in principle. Ed designed the interface to be
simple, requiring no prior knowledge of the data to be able to read them.
Hence, you get _everything_ in one call. This eliminates the use of a
(GUI?) fornt-end interface to have the user decide what to grab. However,
there is no reason why it should not be possible to request only specific
data (other than manpower to implement it).

Let me suggest the following: If you _really_ only want to be able to
request certain quantities, write an IDL wrapper for now to "fake" it.
Implement a call along the line oyu have in your question, and then have
an IDL routine read the data (whole record) and only return the quantity
you wanted. So in you software you already have the look and feel you
want, and we can worry about the implementation later. I say
later, because the "matrix" will be more important for you right now.
Admittedly, you still read all the data this way (at a speed penalty).

I would be curious about the difference in speed between reading all and
only one variable. Because if that difference is negligible, then wrapping
it rather than extending the DLM might be the answer.


Cheers,

Joerg-Micha



+-------------------------------------------------------------------------+
   Jörg-Micha Jahn, Ph.D.

   Space Science Department               Phone:   (210) 522-2491
   Southwest Research Institute           FAX:     (210) 520-9935
   6220 Culebra Road                      E-mail:  jm...@sw...
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